This invention relates to a method and apparatus for error correction in a position sensing circuit and, more particularly, to a method and apparatus for error correction in a DSCG (digital sine-cosine generator)-type position sensing circuit that employs a resolver or the like as the position sensing circuit.
A resolver or Inductsyn (trade name) is widely employed as a position sensor in servo control. The resolver has primary windings (stator windings) constituting a cosine excitation coil and sine excitation coil so arranged that a phase difference of .pi./2 exists between the two coils, and a secondary winding (rotor winding) that revolves with respect to the primary windings in accordance with the rotation of a motor or the like.
If primary voltage signals E.sub.a, E.sub.b given by the equations EQU E.sub.a =E.sub.c0 .multidot.cos.alpha..multidot.sin wt (1) EQU E.sub.b =E.sub.s0 .multidot.sin.alpha..multidot.sin wt (2)
are impressed upon the respective cosine and sine excitation coils comprising the primary windings when the turning angle (referred to as the mechanical angle) of the secondary winding is .theta., then a secondary voltage E.sub.o is induced in the secondary winding. This secondary voltage is given by the following equation: ##EQU1## Here .alpha. is an electrical angle which will be described later, and E.sub.s1, E.sub.c1 are values proportional to E.sub.s0, E.sub.c0, respectively. These constants depend upon the degree of electromagnetic coupling between the secondary winding and each of the excitation coils. If E.sub.s1 =E.sub.c1 =E.sub.1, the secondary voltage E.sub.o will be given by EQU E.sub.o =E.sub.1 sin (.theta.-.alpha.).multidot.sin wt (4).
Accordingly, if the electrical angle .alpha. is so varied as to make the quantity (.theta.-.alpha.) equal to zero (.theta.-.alpha.=0), and if the change in the value of the electrical angle .alpha. is monitored, then it is possible to sense the position or amount of movement of a moving body such as a motor.
The DSCG-type position sensing system operates on the foregoing position sensing principle. A position sensing circuit based on the DSCG position sensing method includes a smoothing circuit for smoothing the secondary voltage signal E.sub.o, a voltage-frequency converter for generating a pulse train of a speed which is in accordance with the output voltage of the smoothing circuit, a primary voltage generating circuit for so varying the electrical angle .alpha. in accordance with the number of pulses in said pulse train as to make the quantity (.theta.-.alpha.) equal to zero, the circuit accomplishing this by generating the primary voltage signals E.sub.a, E.sub.b through digital processing, and a counter circuit for counting the number of pulses in the pulse train. The counter circuit is adapted to count up or to count down the pulses in accordance with the direction of movement of the moving body, so that the value of the count within the counter will represent the current position of the moving body.
In a DSCG position sensing system, it is obvious from equation (3) that E.sub.c1 and E.sub.s1 must be equal in order to avoid an error in the measurement of position. However, E.sub.c1 and E.sub.s1 will not be equal in equation (3) even if E.sub.c0 and E.sub.s0 are made equal in equations (1) and (2). This is because the degree of electromagnetic coupling between the cosine excitation coil and the secondary winding, which is the sensing coil, is different from the degree of electromagnetic coupling between the sine excitation coil and the secondary winding.